The present invention relates to a new and improved method for removing solder from the drill holes or bores of component-free printed circuit boards coated with solder.
During the production of printed circuit boards for printed circuits there is frequently applied, as a final step in the fabrication process, a protective solderable coating to the printed circuit boards, so that, among other things, there can be improved their storage retention properties or shelf life. The possibility exists of coating the conductor tracks of the printed circuit boards in accordance with one of the state-of-the-art methods, such as immersion soldering, splash bath soldering and so forth, with a solder layer of a predetermined thickness. As a general rule, however, when performing these solder-coating methods also the drill holes or the like and, in particular, the through-metallized holes of the printed circuit boards are filled with solder which, at the latest, must be again removed prior to mounting the electrical components or elements at the printed circuit boards. Under the expression "solder" as used in the context of this disclosure there is to be understood all those solder alloys which can be conventionally employed during the soldering of printed circuit boards. Equally, at times in this disclosure reference may be made to "de-tinning" and such term is to be used in its broader sense as encompassing not only the removal of solder containing tin but other types of solders which might be beneficially employed in conjunction with printed circuit boards.
To free the drill holes from excess solder there have become known to the art different methods and apparatuses. Thus, for instance, there has been proposed a so-called solder centrifuge which receives a printed circuit board coated with a flux agent, immerses such in a vat or receptacle containing a molten solder and after removal therefrom rotates the plate or printed circuit board in such a manner that, the excess solder is removed out of the holes by the action of the centrifugal force. Such type of equipment requires an appreciable mechanical expenditure.
A further method known to the art has been designated as the so-called hot air solder leveling process. Significant in this regard is the publication "Proceedings of the First Printed Circuit World Convention", Volume 2, June 5-8, 1978, the article of D. Schoenthaler, entitled "Solder Coating Thickness Considerations for Hot Gas Solder Leveling", pages 2.4.1 to 2.4.8. With this technique the printed circuit boards or plates are attached in a holder, immersed in a molten solder bath and directly after removal therefrom contacted at both sides or faces with hot compressed air, so that there is here likewise accomplished freeing of the drill holes from the excess solder. Practical experience has shown, however, that this technique is not devoid of its problems. With this method, as far as the resultant solder layer thickness in the drill holes is concerned, there is decisive, apart from the solder composition, the solder bath temperature, the residence time in the solder bath and the withdrawal speed, also the angle of the hot air nozzles with respect to the plane of the printed circuit boards, their spacing from the printed circuit boards as well as the pressure and temperature of the hot air. In particular, the optimum setting of the last four-mentioned parameters can be particularly difficult and quite different depending upon the type of printed circuit board which is being processed. Also, with this state-of-the-art method the danger exists that the drill holes will be freed to a greater extent than permissible, at the region of their edges, from the solder by the action of the compressed air and at these locations undesirable oxidation of the conductor material will occur. Furthermore, during the blowing-out operation there must be expected an uncontrolled flying about of solder spatters which, for instance, again could undesirably resettle in the drill holes. Furthermore, with such prior art equipment measures must be provided for removal of the noxious metallic vapors which are circulated during the blowing operation. Thus, on balance, an apparatus operating according to this state-of-the-art method, requires a relalively large amount of space and equipement expenditure, so that it is not readily possible to incorporate such equipement in an extensively automated production line. The mere requirement of providing a source of hot compressed air and the corresponding higher energy consumption, renders questionnable, under circumstances, the economically justifiable employment of such method.